A variety of sensors exist for converting real-world properties into electrical signals that can be manipulated for various purposes. For example, an electret condenser microphone (ECM) converts acoustic signals (e.g., such as a person's voice) into analog electrical signals that can be provided to an associated device such as a personal computer, cellular phone, wireless phone, conference phone, voice recorder, and so on. Many such associated devices include a well-known analog input interface to receive the electrical signals that embody the acoustic signal as captured by one or more ECMs.
For example, FIG. 5 shows a conventional ECM 50 connected to an associated device 500 via a signal line 10. ECM 50 includes an ECM capsule 51 and a junction field-effect transistor (JFET) 52. The output of ECM capsule 51 is connected to the gate of JFET 52, which is coupled between a voltage supply (VDD) and ground potential. ECM capsule 51 converts acoustic signals (e.g., such as a person's voice) into electrical signals, and JFET 52 amplifies the electrical signals received from ECM capsule 51 to produce a low-impedance analog output signal. Although JFET 52 consumes valuable circuit area and requires a connection to VDD, its amplifying function reduces the output signal's susceptibility to undesirable external interference such as electric fields and magnetic fields. Thus, without JFET 52, the high-impedance signal output from ECM capsule 51 is very susceptible to such interference.
The analog output signal (OUT) is provided from ECM 50 via signal line 10 and is received into device 500 via a well-known input interface 520 that includes a bias resistor (Rbias) connected between VDD and signal line 10. A coupling capacitor (Cin) blocks unwanted DC components of the analog input signal from being transmitted to circuitry 510 within device 500. VDD is a standard operating voltage (e.g., such as 1.8 volts or 3.3 volts), and Rbias is sized to bias the input signal line 10 at approximately 1.5 volts. Typically, the input signal line 10 is responsive to voltage differentials of a few tens to a few hundred milli-volts in the analog output signal OUT, and can provide between 200-300 uA of current to ECM 50. Because input interface 520 is configured to receive analog signals from ECM 50, input interface 520 is sometimes referred to as an ECM-compatible interface. Indeed, to ensure compatibility of device 500 with external microphones such as ECM 50, input interface 520 is typically configured to operate as described above.
In many applications, it is desired to improve the sound quality of acoustic signals provided by microphones such as ECM 50. For example, a digital signal processing (DSP) based circuit can be connected between the ECM and the associated device and configured to processes the electrical signals provided by the ECM using various techniques such as noise reduction and directional sensitivity to improve sound quality. The DSP based solution typically includes an analog-to-digital converter (ADC), a DSP circuit, and possibly a digital-to-analog converter (DAC). The ADC converts the analog signals received from the ECM into digital signals that can be processed by the DSP circuit to perform one or more desired functions such as noise reduction and/or directional sensitivity. The DAC converts the digital signals output from the DSP circuit back into analog electrical signals that are compatible for input to ECM input interface 520 of device 500.
The various components of the DSP engine (e.g., the DSP circuit, the ADC and DAC circuits, and other associated circuitry) are relatively complex, require a significant amount of silicon area, and consume significantly more power than is typically available from the signal line 10 connected to an ECM-compatible analog input such as input interface 520. As a result, the DSP engine is typically formed as a separate IC chip that includes its own power connections to VDD and includes input terminals to receive electrical signals embodying acoustic signals from a separate microphone circuit, which undesirably limits the ability for such systems to be miniaturized and/or deployed in low-power applications. Accordingly, there is a need for a microphone system that can implement signal processing functions such as noise reduction and directional sensitivity and yet be housed in module that is significantly smaller and requires significantly less power than conventional acoustic processing systems.
Like reference numerals refer to corresponding parts throughout the drawing figures.